Heat treatment of alloy cast iron



Dec. 3; 1968 R GREENWOOD ET AL 3,414,442

HEAT TREATMENT OF ALLOY CAST IRON Filed June 10, 1966 0 a I l 0 4 8 l2I6 Z0 24 7/71: f/oqes 1N VEN TORS United States Patent "ice 3,414,442HEAT TREATMENT OF ALLOY CAST IRON Roger Kearsley Greenwood, Pedmore,Stourbridge, and

Cedric George Westwood, Kidderminster, England, assignors to TheInternational Nickel Company, Inc., New York, N.Y., a corporation ofDelaware Filed June 10, 1966, Ser. No. 556,759 Claims priority,application Great Britain, June 17, 1965, 25,706/ 65 12 Claims. (Cl.148-141) This invention relates to a process for the heat treatment ofcast iron and, more particularly, to the heat treatment of alloyed whitecast iron to enhance hardness, toughness, and impact fatigue life.

It is generally known that white cast iron alloyed wit nickel andchromium has superior resistance to both wear and abrasion. Thesesuperior properties are largely due to the presence in themicrostructure of massive carbides in a martensitic-austenitic matrix.If, in addition to wear and abrasion resistance, strength, toughness,and impact fatigue life are of importance, it is preferred to usealloyed white cast irons in which the carbide phase is discontinuousrather than those in which the carbide phase is present as a continuousnetwork since a continuous carbide phase provides a continuous path forpropagation of fractures. However, the improved strength, toughness, andimpact fatigue life of alloyed white cast irons having a discontinuouscarbide phase is obtained at some loss in hardness in the iron as-cast.

Heretofore, the art has endeavored to increase the hardness of alloyedcast irons such, for example, as the iron described in US. Patent No.2,662,011, through heat treatment. This heat treatment consisted ofheating the iron to a temperature high enough to cause some or all ofthe austenite in the matrix to transform to martensite or bainite. Suchtemperatures are generally in excess of 450 C. and, typically, are about750 C. The cast iron is held at the transformation temperature for about8 hours and is, thereafter, air-cooled to room temperature.

It has now been discovered that if the rate at which nickel-chromiumalloyed white cast iron is cooled from the transformation temperature toat least about 200 C. is carefully controlled, a marked increase inhardness, toughness, and impact fatigue life results.

It is an object of the present invention to provide a process for heattreating cast irons which greatly enhances their hardness, toughness,and impact fatigue life.

Another object of this invention is to provide a process for heattreating an alloyed martensitic white cast iron which greatly enhanceshardness, toughness and impact fatigue life.

The invention also contemplates providing an alloyed, white cast ironhaving greatly enhanced hardness, toughness, and impact fatigue life inthe heat treated condition.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawing in whichthe figure depicts the time-temperature curves for ball castings 60millimeters in diameter made of an alloy cast iron cooled in the furnaceand cooled at a predetermined, programed rate.

Generally speaking, the present invention contemplates a heat treatmentprocess in which an alloyed white cast iron having the followingcomposition: about 2.8% to 3.7% carbon, about 0.5% to 3.0% silicon,about 0.2% to 1.5% manganese, about 4% to 8% nickel, about 4% to 15%chromium, and the balance iron with residual impurities, is heated to atemperature at least sufficient to transform some or all of the retainedaustenite, present in the as-ca'st condition, to martensite or bainite,e.g., about 450 C. to about 750 C. or about 800 C., maintained at thistemperature for a peroid sufiicient to trans- 3,414,442 Patented Dec. 3,1968 form most of the retained austenite to martensite or bainite,slowly cooled to about 200 C. or lower at a rate not exceeding about 50C. per hour and, thereafter, cooled at any convenient rate.

After heat treatment, in accordance with this invention, the cast ironsnormally have a microstructure consisting of a discontinuous carbidephase in a matrix of tempered martensite or bainite with little or noaustenite. In addition, cooling in accordance with this invention alsoserves to avoid internal stresses.

In carrying the invention into practice, it is advantageous to treat analloyed white cast iron containing about 2.8% to 3.2% canbon, about 1.5%to 2.0% silicon, about 0.4% to 0.6% or 0.8% manganese, about 5.5% to6.5% nickel, about 7.5% to 9.0% chromium, and the balance iron withresidual impurities by heating to about 750 C. to about 800 C. for about8 hours, cooling to about 200 C. or lower at a rate not exceeding about30 C. per hour and, thereafter, cooling below about 200 C. at anyconvenient rate.

For the purpose of giving those skilled in the art a betterunderstanding of the invention, the following illustrative example isgiven:

Example I An iron containing 3.1% carbon, 1.7% silicon, 0.7% manganese,5.7% nickel, 7.6% chromium was cast into balls 60 millimeters indiameter, which were then heated to 750 C. for 8 hours. A first group ofballs was air cooled to room temperature; a second group of balls wasfurnace cooled in accordance with the curve identified by the legend,Furnace Cool, shown in the accompanying figure; a third group of ballswas cooled at a programed, uniform rate of about 23 C. per hour inaccordance with the invention down to about 200 C. as shown by the curveidentified by the legend, Program Cool, in accordance with theaccompanying figure. The first group of balls, air cooled according tothe prior art, was found to have a hardness of about 700 DPN, andfractured after an average of about 4000 drops in a test wherein theballs were repeatedly dropped from a height of about 21 feet onto ananvil. The second group of balls, furnace cooled at a cooling ratefaster than that contemplated in accordance with the invention, had ahardness of about 700 DPN and fractured after an average of about 5070drops. The third group of balls, cooled at a controlled rate inaccordance with this invention, had a hardness of about 780 DPN and didnot fracture after an average of more than 10,000 drops.

The present invention is particularly applicable to the heat treatmentof cast iron balls for use in grinding mills.

Although the present invention has been described in conjunction withadvantageous embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A heat treatment process for improving hardness, toughness, andimpact fatigue life of alloyed white cast irons comprising about 2.8% to3.7% carbon, about 0.5% to 3.0% silicon, about 0.2% to 1.5% manganese,about 4% to 8% nickel, about 4% to 15 chromium, balance iron withresidual impurities, which comprises:

(a) heating the cast iron to a temperature of about 450 C. to about 800C. in order to transform the retained austenite present in the as-castcondition;

(b) maintaining this temperature for a period sufficient to transformsubstantially all the retained austenite;

(c) slowly cooling the cast iron to about 200 C. at a rate not exceedingabout 50 C. per hour.

2. A process of heat treating cast irons in accordance with claim 1wherein the cast iron is cooled at a rate not exceeding about 30 C. perhour.

3. A process for heat treating cast irons in accordance with claim 1,wherein the cast iron is heated to about 750 C. to about 800 C.;maintained at this temperature for about 8 hours; and then cooled toabout 200 C. at'a rate not exceeding about 50 C. per hour.

4. A process for heat treating cast irons in accordance with claim 3wherein the cast iron is cooled at a rate not exceeding about 30 C. perhour.

5. A process for heat treating cast irons in accordance with claim 1wherein the cast iron comprises: about 2.8% to 3.2% carbon, about 1.5%to 2.0% silicon, about 0.4% to 0.8% manganese, about 5.5% to 6.5%nickel, 7.5% to 9.0% chromium, balance iron with residual impurities.

6. The process for heat treating cast irons in accordance with claim 5in which the manganese content of the cast iron is about 0.4% to 0.6%.

7. A process for heat treating cast irons in accordance with claim 5wherein the iron is cooled at a rate not exceeding about 30 C. per hour.

8. A process for heat treating cast irons in accordance with claim 5wherein the iron is heated to about 750 C. to about 800 C.; maintainedat this temperature for about 8 hours; and then cooled to about 200 C.at a rate not exceeding about 50 C. per hour.

9. A process for heat treating cast irons in accordance with claim 8wherein the cast iron is cooled at a rate not exceeding about 30 C. perhour.

10. A process for heat treating cast irons in accordance with claim 1,wherein the cast iron is heated to about 4 750 C.; maintained at thistemperature for about 8 hours; and then cooled to about 200 C. at a ratenot exceeding about C. per-hour.

11. A process for heat treating cast irons in accordance with claim 10wherein the cast iron is cooled at a rate not exceeding about 30 C. perhour.

12. A new article of manufacture produced in accordance with the processof claim 8.

References Cited UNITED STATES PATENTS 1,988,910 1/1935 Merica et a1.148-35 2,353,688 7/1944 Burgess. 2,516,524 7/ 1950 Millis -123 2,530,54511/1950 Sefing. 2,646,375 7/1953 Sefing. 2,662,011 12/ 1953 Gagnebin etal.

FOREIGN PATENTS 727,061 1955 Great Britain.

OTHER REFERENCES International Nickel Co. Ltd., Engineering Propertiesand Applications of Ni-Harid Martensitic White Cast Irons, p. 12, 1965.

International Nickel Co. Ltd., Ni-Hard Iron- Type 4, pp. 2 and 3, 1966.

Transactions of American Foundrymens Society, vol. 67, 1959, pp.242-256.

Physical and Engineering Properties of Cast Iron, 1960, British CastIron Research Association, Birmingham, England, pp. 267-270 and 495-499.

CHARLES N. LOVELL, Primary Examiner.

1. A HEAT TREATMENT PROCESS FOR IMPROVING HARDNESS, TOUGHNESS, ANDIMPACT FATIGUE LIFE OF ALLOYED WHITE CAST IRONS COMPRISING ABOUT 2.8% TO3.7% CARBON, ABOUT 0.5% TO 3.0% SILICON, ABOUT 0.2% TO 1.5% MANGANESE,ABOUT 4% TO 8% NICKEL, ABOUT 4% TO 15% CHROMIUM, BALANCE IRON WITHRESIDUAL IMPURITIES, WHICH COMPRISES: (A) HEATING THE CAST IRON TO ATEMPERATURE OF ABOUT 450* C. TO ABOUT 800* C. IN ORDER TO TRANSFORM THERETAINED AUSTENITE PRESENT IN THE AS-CAST CONDITION; (C) SLOWLY COOLINGTHE CAST IRON TO ABOUT 200* C. AT A RATE NOT EXCEEDING ABOUT 50% C. PERHOUR.